Vaccines are one of the most cost effective medical treatments in modern civilization. Antibody responses are critical components of protective immune responses to many viruses, and almost all currently licensed vaccines work on the basis of protective antibodies. The vast majority of neutralizing antibody responses to pathogens are CD4 T cell help dependent. Follicular helper (Tfh) cells are the CD4 T cells uniquely specialized for B cell help. Our lab specializes in the study of Tfh cells. A major goal o vaccination against viruses, bacteria, or parasites is to optimize long-term high-affinity antibody responses, but the best strategy for achieving this result is not clear. We have found that the intracellular adaptor SAP and the SLAM family receptors are critical for germinal centers (GC) and long term antibody responses to virtually all pathogens because of their importance in Tfh cell and T:B interactions. These signals are conserved in humans, and mutations in Sh2d1a, the gene encoding SAP, are the cause of a severe human immunodeficiency that, when left untreated, frequently results in death associated with a variety of infections. In the initial fundng period of this R01 Tfh cells were generally unrecognized. We helped establish that Tfh cell are a distinct type of differentiated CD4 T cell, that required Bcl6 for their differentiation, that are required for GCs and most B cell memory, and that depend on signals from both DCs and B cells as APCs. We recently made the surprising finding that SAP is essential to block powerful negative signaling by the SLAM family receptor LY108. However, the mechanism remains to be shown. We now propose to determine the mechanism. Furthermore, positive signaling in Tfh function is likely mediated by several of the SLAM family members, including LY108. Therefore, we will determine which SLAM family receptors provide positive signals. Finally, much of our efforts were to establish that Tfh differentiation is a multistage, multifactorial process. SAP is required for full Tfh differentiation, but is not required for early Tfh differentiation, and thereore we continue to intensively investigate early signals required for Tfh differentiation in vivo and i vitro.